The present invention relates to an elapsed time recording device for cumulatively recording the period during which an electrical appliance is turned on.
Electrical appliances, such as visual display units for computer systems, are turned on and off many times after installation. Therefore, the cumulative period or "Power On Hours" during which such an electrical appliance is actually turned on can be much shorter than the period measured from installation. If the appliance fails after installation, the time to failure can be difficult to determine. This can be extremely frustrating to the purchaser because the warranty period applicable to the appliance may expire before the appliance has been used significantly, or even before the appliance has been turned on after installation.
Conventionally, elapsed time recording devices have been fitted to some electrical appliances with a view to determining the time to failure of the appliances during long term reliability testing. Examples of conventional elapsed time recording devices include mercury timers and motor driven meters. Mercury timers are generally two terminal devices comprising a column of mercury that increases in length as a function of the time during which electrical current passes through it. However, mercury timers are relatively inaccurate. Furthermore, mercury timers are open to fraudulent use because removal of one of the terminals will prevent advance of the column. Therefore, mercury timers are not generally suitable for indicating whether or not the warranty period applicable to an electrical appliance has expired based on the Power On Hours of the appliance. Motor driven meters are relatively expensive because of their mechanical complexity and are therefore unsuitable for use as elapsed time recording devices for electrical appliances produced in high volumes.